The present invention relates to improvements to a measuring device for the measurement of the relative position of two objects, of the type having a scale which defines a graduation and at least one reference mark, absolutely allocated to the graduation, as well as a scanning unit which operates to scan the scale and to generate reproducible scanning pulses which act upon a counter. Such measuring devices typically include at least one scanning plate in the scanning unit for scanning the graduation and the reference marks of the scale, and the separation between the scanning plate and the scale in the direction travelled by the light beam is held at a predetermined spacing.
In such measuring arrangements the reference marks are typically used to generate electric control pulses which can be used in various ways. For example, such control pulses can be used to set the counter to a zero position, to load a preselected position value into the counter at the commencement of the measuring operation, and to monitor the measuring value against interfering impulses. Furthermore, such electric control impulses can be used to act upon and control a control arrangement engaged on the outlet side.
In code measuring systems, of the type which do not include reference marks, on occasion several graduation tracks with differing grid constants are used. In this case, it is a known practice to provide differently sized spacings between the graduation tracks and the associated scanning fields. In this system, it is the grid constants of the graduation tracks and the wavelength of the reading light of the illuminating arrangement that determine the appropriate spacings. See, for example DE-OS 15 48 874.
German patent DE-PS 29 52 106 describes an incremental angle measuring system in which a scale is provided which includes a measuring graduation and a number of reference marks, each of which defines a unique line group distribution. The individual reference marks are scanned by respective scanning fields included in a scanning unit, and each scanning field of the scanning unit is clearly and unambiguously allocated to a respective one of the reference marks. This is because the allocated scanning field defines the same line group distribution as that of the associated reference mark. Because of the irregular line group distributions, the distance between the scale and the scanning plate is preferably at most about 4d.sup.2 /.lambda., where d is the width of the narrowest line of the line graduation of the reference mark and .lambda. is the center of gravity wavelength of the light. Typically, this spacing is desirable for clear and error free scanning of the reference marks.
In addition, it is known that for the scanning of a regular, periodic incremental graduation of a scale, the scanning distance between the scale and the scanning plate does not need to be maintained at a single definite spacing. Rather, a number of differing spacings are possible. When the graduation of a scale is traversed by light in a parallel beam path, diffraction patterns of the graduation of the scale are generated which can be scanned with a scanning graduation of like grid constant. Only at certain planes behind the graduation plane of the scale does the interference associated with the diffraction patterns provide an optimal pattern for scanning. In general, for a graduation with a grid constant P.sub.M and a center of gravity wavelength .lambda. of the light, the optimal scanning planes are separated from the graduation plane of the scale by a distance equal to n.multidot.P.sub.M.sup.2 /.lambda. (n=0, 1, 2, . . .). Thus, optimal scanning signals can be generated at a separation equal to n.multidot.P.sub.M.sup.2 /.lambda. between the graduation plane of the scanning plate and the graduation plane of the scale (see Machine Shop Magazine, April, 1962, p. 208). Larger spacings between the scale and the scanning plate bring advantages in that the measuring device is less sensitive to mechanical influences, such as for example, processing chips or shavings. In the event of small spacings between the scanning plate and the scale, such processing chips or shavings can more readily become jammed between the scale and the scanning plate and can lead to damage to the graduations of the scale and the scanning plate. Furthermore, when larger separations are used the scale and the scanning plate can more readily be cleaned in the event of fouling. Yet another advantage of larger separations between the scale and the scanning plate arises from the fact that spacing tolerances are also greater with these greater spacings. For this reason, lower demands can be placed on the precision of guidance of the scanning plate with respect to the scale. Furthermore, in the event larger separations are used between the scale and the scanning plate, the periodic scanning signals generated in connection with the incremental graduation of the scale have a more sinusoidal signal form, so that the signal period of the scanning signal can better be subdivided by interpolation.
In the past, the utilization of very large spacings between the scale and the scanning plate in the scanning of the incremental graduation has been possible without interposed imaging optics only in measuring devices which include no reference marks on the scale. This is because for the clear scanning of the reference marks, as set out above, a predetermined small distance between the scale and the scanning unit must not be exceeded.